The articulation of the superior heads of the ulna and radius with the humerus forms the elbow-joint. The ulna is an irregularly shaped bone. The upper head has two processes, the olecranon behind, and the coronoid in front ; the cavity formed by these two processes is called the greater sigmoid cavity, because it is supposed to resemble the Greek letter sigma (s). The trochlear of the humerus fits into the sigmoid cavity, and thus forms a hinge-joint. A projection at the lower end of the ulna is called the styloid process.

Fig. 33.—Ulna (Left).

1, Olecranon; 2, great sigmoid cavity ; 3, coronoid process ; 4, styloid process.

Fig. 34.—Radius (Left).

1, Bicipital tuberosity; 2, styloid process.

The radius is the outer bone of the forearm. The upper head is small and button-shaped, with a rim round it. It rotates on the capitellum of the humerus, and forms a pivot-joint. The inner side of the rim rolls in a small cavity in the outer side of the ulna, called the lesser sigmoid cavity, and forms the superior radio-ulnar joint. The neck is surrounded by a ligament in which it rotates, and which is attached to the ulna, in front of and behind the lesser sigmoid cavity. Below the neck on the inner side is a prominence called the bicipital tuberosity, into which the lower end of the biceps muscle is inserted. At the lower end there is another small sigmoid cavity, with which the lower end of the ulna articulates, and forms the inferior radio-ulna joint. The radius has a projection at the lower end called the styloid process.